Numerical control apparatuses (CNCs) for controlling stations by servo motors, which conventionally are pneumatically or hydraulically controlled, are now widely used to control a transfer line, and although these CNCs are known as a one axis CNC and control each station independently, a cooperative operation is needed thereamong.
For example, when a drilling operation is carried out at a first station and a tapping operation is carried out at a second station, the second station cannot carry out the tapping operation until the machining at the first station has been completed and the workpiece has been transferred to the second station.
An example of the working of this cooperative operation will be explained with reference to FIG. 2, which shows an example of a conventional cooperative operation system for CNCs. As shown in the figure, a one axis CNC 30 is composed of a CNC control unit 31, a RAM 33 in which an I/O signal is stored, a programmable machine controller (PMC) 36 for controlling the I/O signal, and an I/O circuit 37 having a driver and a receiver.
The one axis CNC 40 also shown in FIG. 2 has the same arrangement as that of the one axis CNC 30. A transfer line is actually composed of a plurality of these one axis CNCs, but a case in which only two one axis CNCs are used will be described here for simplicity.
A cooperative operation between these one axis CNCs 30 and 40 is carried out between the built-in PMCs 36 and 46 through the I/O circuits 37 and 47. An example of this is a case in which a machining completion signal of the one axis CNC 30 is transferred to the one axis CNC 40.
The receiver of the I/O circuit has a slow signal transfer speed, taking into consideration a noise margin and contact chatter and the like, because the receiver receives signals from limit switches and the like. Further, the driver also has a slow signal transfer speed, because the driver is designed by taking into consideration a required drive power and the like.
Therefore, an undue time is needed to transfer the machining completion signal from the one axis CNC 30 to the one axis CNC 40, and an answer back signal thereof from the one axis CNC 40 to the one axis CNC 30. Also, since the answer back signal is generated by the PMC 46 and confirmed by the PMC 36, the sequence program of each of the PMCs 36 and 46 is complex.
The above refers only to the case in which the machining completion signal is transferred from the one axis CNC 30, but many signals must be transferred and received when one table is controlled by the one axis CNCs 30 and 40, and thus the conventional system sometimes cannot carry out the required processes within a predetermined time.